The Senedd constituencies and electoral regions (Welsh: Etholaethau a Rhanbarthau etholiadol Senedd Cymru) are the electoral districts used to elect Members of the Senedd (MS; Welsh: Aelodau'r Senedd or AS) to the Senedd (Welsh Parliament; Welsh: Senedd Cymru), and have been used in some form since the first election of the then National Assembly for Wales in 1999. New boundaries were introduced for the 2007 elections and currently consist of 40 constituencies and 5 regions. Voting last took place in all districts in the 2021 Senedd election.
The constituencies were created with the boundaries of the Welsh constituencies of the House of Commons of the Parliament of the United Kingdom (Westminster), as they were in 1999. The new boundaries were also used for the 2010 United Kingdom general election. Therefore, between the 2007 Assembly election and the 2010 United Kingdom general election, the two sets of constituencies, Assembly and Westminster, had differing boundaries.
Three constituency names, Conwy, Caernarfon, and Meirionydd Nant Conwy, have become historic, and the new boundaries define three constituencies with new names: Arfon, Dwyfor Meirionnydd, and Aberconwy. Generally, the new boundaries define each constituency taking account local government ward boundaries, and define constituencies close to equal in terms of the sizes of their electorates.
Unlike Westminster constituencies, Senedd constituencies are grouped into electoral regions, and an additional member system is used to elect four additional Members of the Senedd from each region, in addition to the MSs elected by the constituencies. The Electoral Regions boundaries were based upon the pre-1999 European Parliament constituencies. At each general election of the Senedd, each elector has two votes, one constituency vote and one regional party-list vote. Each constituency elects one Member by the first past the post (single-member district plurality, SMDP) system, and the additional Senedd seats are filled from regional closed party lists, under the D'Hondt method, with constituency results being taken into account, to produce a degree of proportional representation for each region. Altogether, 60 Members are elected from the 40 constituencies and five electoral regions, creating a Senedd of 40 constituency MSs and 20 additional MSs. Every constituent is represented by one constituency Member and four regional Members.
- List of Members at the Senedd Cymru website